Infrared, red, orange, yellow and green light emitting diodes have been already produced and used on a large scale in various fields of applications. However, blue light emitting diodes have not been put to practical use yet. This is owing mainly to the restriction of material.
General requirements demanded for the material of the blue light emitting diodes are
(1) the band gap of the material should be wider than 2.5 eV,
(2) the material should have direct transition type band structure; an electron at the bottom of the conduction band can transit to the top of the valence band without generation on absorption of phonon,
(3) the pn-junctions should easily be fabricated,
(4) large substrates should easily be produced,
(5) the device processes should be facile,
So far gallium nitride (GaN), silicon carbonate (SiC), zinc selenide (ZnSe), zinc sulphide (ZnS), gallium aluminum nitride (GaAlN), and zinc sulfo-selenide (ZnSSe) have been proposed as the materials for blue light emitting diodes which satisfy some of the requirements.
Among them, zinc selenide (ZnSe) is a promising semiconductor, because ZnSe has a wide band gap of 2.7 eV and it is a direct transition type semiconductor. However, ZnSe has drawbacks that growing a large bulk singlecrystal ZnSe with few defects is difficult and a p-type ZnSe cannot be produced yet.
High pressure Bridgman method, sublimation method, iodide transportation method, growth-in-solution method, Piper's method and so forth have been proposed to grow bulk ZnSe singlecrystals. What makes it difficult to grow ZnSe singlecrystals is that ZnSe should be grown at high temperature under high pressure, since ZnSe vehemently sublimes at high temperature. In order to melt material solid ZnSe suppressing sublimation, man must heat the material solid ZnSe above 1520.degree. C. under pressure more than 50 atm (5 MPa).
The Inventors had proposed an improvement of sublimation method for growing a ZnSe singlecrystal for substrates. This method made use of sublimation of ZnSe in an ample having two spaces and a neck connecting the two spaces in a gradient of temperature. The method comprised the steps of storing ZnSe polycrystal in one space of the ample, sealing the ample, heating the ample in a furnace having a gradient of temperature so as to position the solid ZnSe at the higher temperature zone, subliming the solid ZnSe little by little into a ZnSe gas and recrystallizing the gas ZnSe into a ZnSe crystal at the lower temperature zone. Since the ZnSe gas was transported from the higher temperature zone to the lower temperature zone through the neck by the gradient of temperature, this method was called a transportation method. The ZnSe singlecrystal obtained by their improved method was an insulator with high resistivity. However, an n-type semiconductor ZnSe singlecrystal with lower resistivity can be obtained by heat-treating the insulating ZnSe singlecrystal in a zinc melt. This method enables us to grow a pretty large n-type ZnSe singlecrystal.
F. Takeda, A. Matsuda, S. Kishida, K. Matsuura and I. Tsurumi: Report of the Faculty of Engineering. Tottori University, vol. 13, p. 56 (1982)
The improvement succeeded in solving one difficulty of ZnSe devices; making a large singlecrystal ZnSe. However, another serious difficulty still remains. Namely a p-type ZnSe cannot be produced yet ever by our improvement. No effective solution has proposed yet concerning the fabrication of p-type ZnSe singlecrystals.
No pn-junction will not be fabricated, if a p-type singlecrystalline ZnSe film cannot be grown on an n-type singlecrystalline ZnSe substrate. Without pn-junction, no blue light emitting diode of ZnSe can be produced.
Man has tried various methods, e.g. a solution growth method or a vapor phase deposition method, for fabricating p-type ZnSe singlecrystals.
J. Nishizawa et al. reported that they had succeeded in fabricating a p-type bulk ZnSe singlecrystal by a temperature difference method under controlled vapor pressure (TDM-CVP). According to the report, the method comprised the steps of laying ZnSe solid material on the selenium (Se) melt under the saturation vapor pressure of zinc, doping lithium (Li) as a p-type dopant into the selenium (Se) melt, growing a p-type ZnSe singlecrystal at 1050.degree. C. under 7.2 atm (0.72 MPa).
J. Nishizawa and R. Suzuki, J. Appl. Phys. 59(6), 15, (1986) p. 2256
However, they have succeeded only once in growing the p-type ZnSe by the method. Nobody except them has succeeded in growing one by the same method so far. Thus the method has not been confirmed by anybody yet. Since it was very difficult to make a p-type ZnSe bulk crystal, man has tried to make p-type ZnSe film crystals on some proper substrates. Recently trials based on the MOCVD (metalorganic chemical vapor deposition) method or the MBE (molecular beam epitaxy) method have been done.
K. Akimoto, T. Miyajima and Y. Mori: J. Crystal Growth 101 (1990) 1009-1012.
K. Akimoto, T. Miyajima and Y. Mori: Jpn. J. Appl. Phys., vol. 28, No. 4, (1989) L531-534.
These papers reported that a Ga-doped ZnSe (n-type) or an O-doped ZnSe (p-type) was grown on an n-type gallium arsenide (GaAs) substrate by the MBE method. Since they could not obtain a bulk ZnSe singlecrystal of good quality, they used a gallium arsenide (GaAs) singlecrystal as a substrate. They alleged that oxygen (O) plays a role of an acceptor in the ZnSe film they made. The growth of ZnSe was done at 240.degree. C.
M. Migita, A. Taike, M. Shiiki and H. Yamamoto: J. Crystal Growth 101 (1990) p. 835
Mr. Migita et al. reported in the paper that non-doped ZnSe films and N-doped ZnSe had been grown on n-type gallium arsenide (GaAs) substrates by the MOCVD method. The temperature of growth was 250.degree. C. to 450.degree. C. The non-doped ZnSe film turned out to be n-type. They supposed that chloride which had been included a little in the material gas would convert the non-doped ZnSe into an n-type one.
These vapor phase growing methods enable us to grow ZnSe crystal films with low defect density and low impurity concentration at low temperature between 300.degree. C. and 400.degree. C. considerably lower than the melting point of ZnSe by using gallium arsenide wafers as a substrate.
However, even these improved vapor phase growing methods (MOCVD, MBE) have not succeeded in providing a p-type ZnSe of sufficiently good quality. Up to the present there is no blue light emitting diode made from ZnSe which works enough at room temperature.
Unlike these vapor phase growing methods, Singh et al. reported that p-type ZnSe polycrystals had been grown by an electrochemical method.
K. Singh and J. P. Rai: Phys. stat. sol. (a) 99, 257 (1987) p. 257
According to their teaching, the electrochemical method comprised the steps of coating one surface and four sides of a titanium (Ti) plate of 1.3 cm by 1.3 cm with an insulator except the other surface left uncoated, immersing the half-coated titanium plate into a solution of EQU ZnSO.sub.4 +SeO.sub.2
supplying current for 4 hours through the solution and the titanium plate and depositing a ZnSe polycrystal up to a thickness of 5 .mu.m on the uncoated surface of the titanium plate.
The device fabricated by the electrochemical processes has a structure of Ti/ZnSe which is a junction of a metal and a semiconductor. It is not a diode in any way. Singh et al. immersed the Ti/ZnSe plate and a platinum plate as electrodes into a solution of EQU ZnSO.sub.4 +KI+I.sub.2
and measured photovoltaic effect, capacitance of the Ti/ZnSe plate. Furthermore, they observed the response of the Ti/ZnSe plate to X-ray irradiation. They concluded that the ZnSe specimen they made was a p-type polycrystal with considerable photovoltaic effect. However, they could not clarify why the method could make a p-type ZnSe, what could endow the ZnSe with p-type character and what were the acceptors in the ZnSe. The purpose of their attempt was the application of ZnSe semiconductor to photodectors. Although they might succeed in growing a p-type ZnSe polycrystal by the electrochemical deposition method, they failed in making a pn-junction of ZnSe because the substrate was not a ZnSe substrate but a titanium substrate. The Ti/ZnSe specimen could not be a diode, because it lacked a pn-junction. The Ti/ZnSe specimen emitted no light.
Thus, the Ti/ZnSe specimen could be applied neither to a light emitting diode nor a photodiode.
In the case of growing p-type ZnSes by the MOCVD method or the MBE method, a temperature for growth is 300.degree. C. to 400.degree. C., which is still considerably high temperature, although it is far lower than the melting point of ZnSe. High temperature of growth is likely to induce a lot of defects and impurities into a growing ZnSe film which reduce the quality of the film. Furthermore, conventional methods for making p-type ZnSe films require in general such conditions as high pressure, high vacuum or high temperature. These severe conditions demand large scale growing apparatuses.
The purpose of this invention is to provide a method for producing a blue light emitting diode having a pn-junction of ZnSe semiconductor with few defects and impurities at low temperature by a simple apparatus.